Radio-frequency spectroscope



Oct. 25, 1949. E. R. MccoPPlN RADIO FREQUENCY SPECTROSCOPE Filed NOV. 7,1945 *may @mh QMS 29o Patented Oct. 25, 1949 UNITED STATES PATENT OFFICERADIO-FREQUENCY SPECTROSCOPE Everett R. McCoppin, Dayton, OhioApplication November 7, 1945, Serial No. 627,273

8 Claims. (Cl. Z50-20) (Granted under the act of March 3, 1883, as

amended April 30, 1928; 370 O. G. 757) T he invention described hereinmay be manufactured and used by or for the Government for governmentalpurposes, without the payment to me of any royalty thereon.

The present invention relates generally to radio-frequency wave spectrumanalysis and is more particularly directed to a spectroscope adapted forinstantaneous measurement of noise energy within a prescribed frequencyband.

The energy of radio receiver noise impulses caused by static, electricaldevices, ignition systems or other disturbances induced in the anitenna, is normally distributed over a considerable frequency band.Accordingly, apparatus intended for noise measurement must be designedto record or present the frequency as well as the amplitude of noiseenergy within a selected portion of the radio spectrum. With apparatusfor this purpose heretofore known in the art, the measuring techniquewhich is entailed is both tedious and time consuming inasmuch as noiseenergy is measured in a step-wise manner throughout a predeterminedfrequency band.

In View of the foregoing, it is the main object of this invention toprovide a new and improved instrument for noise or signal measurementarranged to display instantaneously the amplitude and frequency of allsignals and noise energy within a predetermined band.

Another object of the invention is to provide a spectroscope whichcontinuously scans and presents all signals and noise energy within arelatively wide radio band.

' Yet another object of this invention is to provide a spectroscope ofthe above character incorporating means for dwelling on a selectedsignal or noise impulse lying within the band.

For a complete understanding of the invention, as well as other objectsand features thereof, reference is made to the following detaileddescription to be read in connection with the accompanying drawing,depicting in a functional block diagram partially in schematic form, one

preferred embodiment of a spectroscope in accordance with the invention.The scope of the invention will be pointed out and defined in theaccompanying claims.

The spectroscope as shown in the drawing mainly comprises an untunedradio-frequency amplier I0, a converter stage I I, a variablehighfrequency oscillator I2, a radio receiver I3, an audio-frequencyrectifier I4, a cathode-ray oscilloscope I5, and a sweep-voltagegenerator I6.

In general terms the behavior of the spectrosoope is as follows: Noiseimpulses are intercepted by a pick-up loop I1 or any other suitableradio-frequency probe and are heterodyned in converter I I with signalsfrom oscillator I2 which are frequency modulated within a prescribedband. Receiver I3 is resonant at a fixed frequency and is associatedwith the output circuit of converter II whereby in the course of acomplete tuning cycle of variable oscillator I2, a beat radio frequencyequal to the resonant frequency of receiver I3 is derived from theoutput of converter II, the amplitude of said beat being proportional tothe level of that noise impulse producing said beat. Thereupon the beatfrequency is translated into an audio-frequency of proportionalamplitude by receiver I3 and is then rectified by audio-frequencyrectifier I4 whose directcurrent output potential corresponds inmagnitude to the input audio-frequency amplitude.

The direct-current output of rectifier I4 is applied to the verticallydeflecting means of cathode-ray oscilloscope I5, thereby deecting thecathode-ray beam in a vertical direction above the base line to anextent in accordance with the amplitude of the beat frequency. Sweepvoltage generator IB provides asawtooth voltage which varies insynchronism with the frequency scan of oscillator I2, said sawtoothvoltage being impressed on the horizontal deflecting means ofoscilloscope I5.

Accordingly, as the cathode-ray beam is deflected vertically to anamount in accordance with the noise impulse, the beam is concurrentlyswept horizontally in accordance with the position of the noise impulsefrequency in the band. Thus, there is presented on the screen ofoscilloscope I5 a panoramic view of the noise impulses within apredetermined band. By Calibrating the horizontal screen base in termsof frequency and the vertical base in terms of amplitude, aninstantaneous measurement of these noise impulses is given. Y

Features of the present invention will become more evident in thefollowing explanation of the schematic circuits of converter Il,variable frequency oscillator I2, and sweep voltage generator I6;

By Way of example, let it be assumed that noise impulses lying Within aband of .1 to 4 megacycles are to be analyzed and that this noise bandis present in the output of untuned amplifier I0.

Variable frequency oscillator I2 is tunable Within a range between 12.1to 16 megacycles. Thus as oscillator I2 scans, a 12 megacyclesdifference beat is produced in the output circuit of converter Ilsequentially for every noise impulse within the noise band beinganalyzed, the other beats being discriminated against by the circuitarrangement. It is to be noted that at any instant a specific frequencyof oscillator I2 is heterodyned with all frequencies in the noise band.In the course of a tuning cycle of oscillator I2, the diiference beatcommon to all frequency combinations in this instance is 12 megacycles.

Receiver I3 which is of standard construction, preferably asuperheterodyne having a high order of sensitivity and selectivity, isfixed at 12 megacycles. Conventional oscillator means are incorporatedtherein for beating with the received radio-frequency signal to producean audiofrequency tone.

Converter II is a standard circuit except that` the input circuitsthereof are untuned to` provide a broad band characteristic. It includesa penta-` grid vacuum tube I8 having the control grid thereof coupled tothe. output of untuned amplifier I through a Xed capacitor I9. The platecircuit of tube I'8 includes a parallel-resonant network peaked at 12megacycles, said network being adapted to reject all other frequencies.

Variable frequency oscillator I2 is of conventional design and includesa triode 2I associated with a resonant circuit having an inductance 22shunted' by a variable capacitor 23. The signal generated by oscillatorI2 is impressed on the injector grid of converter tube I8 through acoupling capacitor 24. Capacitor 23 is of balanced construction, beingvariable through 360 degrees of rotation and attaining maximum capacityevery 90 degreesin the course of rotation. Consequently, the frequencyrange of oscillator I2 is fully transversed in one direction during 0-90and 180-270 in a rotation cycle and in the reverse direction during90180 and 270-360.

Sweep voltage generator I6 comprises a vacuum tube 25 incorporating atriode and diode section. The triode elements of tube 25 are associatedwith a fixed parallel-resonant network 26 in a conventional Hartleyoscillator arrangement. Inductively coupled to network 26 is a coil 21,one side thereof being connected to the diode plate of tube 25, theother side being grounded through fixed capacitor '29. Thus the R.F.voltage across coil 21 is rectified by the diode, the resultantpulsating direct-current being filtered by capacitor 28 and a resistor3|. The output of generator I6 is obtained at terminals 30.

v Connected between parallel-resonant network 2'6 and coil 21, at therespective plate and cathode connections to the triode section of tube25, is a variable capacitor 29, of a construction design similar tocapacitor 23 of variable oscillator I2. It will now be observed thatcoil 21 is capacitatively coupled to network 26 by variable capacitor 29and also, by reason of its proximity, inductively coupled thereto. Thephase of voltage induced inductively in coil 21 is in phase oppositionto that applied capacitatively by variable capacitor 29, hence ascapacitor 29 is increased in value the rectified voltage acrossterminals 30 corre- 'spondingly diminishes as the capacitative transferapproaches the inductive transfer. The arrangement is made such that thecapacitative transfer never exceeds theinductive transfer. Sincecapacitor 2-9 is similar to capacitor 23 it undergoes a full change invalue four times each cycle of rotation.

In the present embodiment variable capacitors 23 and 29 are preferablyof the type having a linear characteristic. Consequently, whencapa'citor 29 is rotated continuously, the voltage api 4 pearing acrossterminals 30 assumes the uniform saw tooth form illustrated by wavepattern 3|.

It is to be noted that the operating frequency of the Hartley oscillatorin sweep voltage generator I6 is not significant and does not controlthe frequency of sawtooth wave 3 I. The frequency of sawtoothv wave 3|'is, however, governed by the rate of rotation of capacitor 29, saidcapacitor being driven by a motor 32 whose shaft is also mechanicallyganged to capacitor 23.

The ganged arrrangement of capacitors 23 and 29 is such that sawtooth 3lcommences at one end of the frequency range of oscillator I2 andsynchronously attains maximum' voltage at the other end of the range,whereupon as the range is traversed in the opposite direction, the sweepvoltage falls to minimum.

Terminals 30 are connected to the horizontally deflecting plates ofoscilloscope I5, while the output of audio-frequency rectifier I4 isconnected to the vertically deflecting plates thereof. As a result, inthe course of the full concurrent 'rotation of capacitors 23 and 29, thecathode-ray beam is swept across the screen in one direction therebydisplaying all noise impulses Within the prescribed band determined byoscillator I2, and the beam then retraces the same band in Vthe reversedirection. l

There exists a distinct advantage in this method. Inasmuch as theluminosity of the trace is an inverse function of beam velocity, theslower the scanning process the more visible is the presentation.However, in order to obtain visual persistence, a scanning speed above apredetermined minimum must be attained. Therefore, by utilizing both theforward and return portions of the trace in the manner of the presentinvention, it is possible to reduce beam" velocity without sacrificingvisual persistence. A feature of the invention resides in the ability ofthe device toy dwell on a selected noise impulse or signal, therebyenabling an individual examination of the impulse. Withconventionalsweepvoltage generators, after the sawtooth circuit istriggered the wave runs its course and it is not possible to freeze theresultant voltage Wave at lany particular level in magnitude. In thepresent invention, however, sweep voltage generator I6 may be maintainedat any selected point in the voltage wave by bringing capacitor 29 to animmediate stop. In consequence, capacitor 23, which simultaneously isbrought to a halt in a corresponding position, fixes the frequency ofoscillator I 2 so that only one noise impulse is displayed on the screenof oscilloscope I5'. This isl accomplished by means of a brake mechanism33 of any suitable design working in conjunction with motor 32.

While the above described embodiment of the invention is shown asemploying capacitors23 and 29- having a linear characteristic,capacitors having non-linear characteristics of any desired form may besubstituted therefor for the purpose of expanding on the screenofoscilloscope l5 a desired portion of the frequency range and contractinga portion of lesser interest.

Superposed over the screen of oscilloscope I5 'is a grid scale 34 whosehorizontalbase line is graduated in terms of frequency and whose-vertical base line is graduated in amplitude, In the present embodiment,grid scale 34 is of 1inear design but obviously where a non-linearpanoramic system is employed the scale may be correspondingly arranged.

The use of a heterodyne system' in the pre's ammesso ent invention bymeans of converter Il and variable oscillator I2 enables a wider noiseband coverage than is possible where the band to be analyzed isdetermined only by the tuning range of receiver I3.

It is, of course, evident that the present invention may be utilizedwith equal eiectiveness for the analysis of continuous wave signals.

An important advantage of the present invention is that it frees anoperator to investigate an "undesired source of noise impulses andpresents instantaneously the results of any noise suppression means hemay introduce at said source.

While there has `been shown what is at present considered a preferredembodiment of the in- Y vention, it is apparent that many changes andmodications may be made therein without departing from the invention,and it is, therefore, intended in the accompanying claims to cover allsuch changes and modifications as fall withinl the true spirit and scopeof the invention.

1 fWhat is claimed is:

M1. A radio-frequency spectroscope for analyzingnoise and signal energycomprising a radiofrequency oscillator including a first variable calpacitator for continuously scanning in frequency within a prescribedband, a converter adapted to heterodyne the frequencies of saidoscillator with the frequencies of energy to be analyzed, a receiverassociated with said converter responsive to a narrow range of beatfrequencies in the output circuit thereof, a cathode-ray oscilloscopehaving horizontal and vertical deecting means, means for generating asweep voltage including a second variable capacitor, the instantaneouscapacitance of which determines the instantaneous value of said sweepvoltage, means gauging said first and second variable capacitors wherebysaid sweep voltage varies in synchronism with the scanning of saidprescribed frequency band, and means for applying the said sweep voltageand said receiver output to said horizontal and vertical deflectingmeans, respectively, of said oscilloscope whereby a panoramic view ofsaid energy is displayed.

, 2. A radio-frequency spectroscope for analyzing noise and signalenergy comprising a radiofrequency oscillator including a first variablecapacitator for continuously scanning in frequency within a prescribedband, a converter adapted to heterodyne the frequencies of saidoscillator with the frequencies of energy to be analyzed, a receiverassociated with said converter responsive to a narrow range of beatfrequencies in the output circuit thereof, a cathode-ray oscilloscopehaving horizontal and vertical deflecting means, means for generating asweep Voltage including a second variable capacitor the instantaneouscapacitance of which determines the instantaneous value of said sweepvoltage, means gauging said first and second variable capacitors wherebysaid sweep voltage varies in synchronism with the scanning of saidprescribed frequency band, means for applying said sweep voltage andsaid receiver output to said horizontal and vertical deilecting means,respectively, of said oscilloscope whereby a panoramic view of saidenergy is displayed, and means in conjunction with said ganging meansfor stopping said first and second capacitors at a selected point intheir movement whereby said oscilloscope dwells on a single energyfrequency.

3. A radio-frequency spectroscope for analyzing noise and signal energycomprising a broadlytuned radio-frequency amplifier for increasing theamplitude of energy in a prescribed band to be analyzed, a radiofrequency oscillator including a first variable capacitor forcontinuously scanning in frequency within said prescribed band. aconverter adapted to heterodyne the fre-y quencies of said oscillatorwith the frequencies of said amplier, a receiver associated with saidconverter responsive solely to a narrow range beat frequencies in theoutput circuit thereof, a rectier in connection with the output circuitof said receiver, a cathode-ray oscilloscope having horizontal andvertical deflecting means, means for generating a sweep voltageincluding a variable capacitor the instantaneous capacitance of whichdetermines the instantaneous value of said sweep voltage, means gaugingsaid first and second variable capacitors whereby said sweep voltagevaries in synchronism with the scanning of said prescribed frequencyband, means for applying said sweep voltage and said rectiiier output tosaid horizontal and vertical deflecting means, respectively, of saidoscilloscope whereby a panoramic view of said energy is displayed, andmeans in conjunction with said gauging means for stopping said first andsecond capacitors at a selected point in their movement whereby saidoscilloscope dwells on a single energy frequency.

4. A radio-frequency spectroscope for analyzing noise and signal energycomprising a, radiofrequency oscillator continuously scanning infrequency, a converter for heterodyning the energy in a prescribed bandwith the output of said oscillator, a flXedly-tuned network responsiveto the output of said converter, means for rectifying the output of saidnetwork, a cathoderay oscilloscope, means responsive to the rectifledoutput for deecting the beam of said oscilloscope along one coordinate,means to deflect the beam of said oscilloscope along the secondcoordinate, and sweep voltage means coupled to said last named means andcomprising an impedance, means for applying two alternating voltages oflike frequency in phase opposition across said impedance to derive aresultant voltage, means for rectifyng said resultant voltage, and meansfor continuously varying the amplitude of one of said alternatingvoltages.

5. A radio frequency spectroscope for analyzing noise and signal energycomprising a radiofrequency oscillator continuously scanning infrequency, for heterodyning the energy in a prescribed band with theoutput of said oscillator, aY fixedly-tuned network responsive to theoutput of said converter, means for rectifying the output of saidnetwork, a cathode-ray oscilloscope, means responsive to the rectiedoutput for deecting the beam of said oscilloscope along one coordinate,means to deflect the beam of said oscilloscope along the secondcoordinate, and sweep voltage means coupled to said last named means andcomprising a radio-frequency oscillator having a tank inductance, apick-up inductance inductively coupled to said tank inductance, meanscoupling said tank inductance to said pick-up inductance to develop avoltage thereacross in phase opposition to the voltage developed byinductive coupling, means for rectifying the resultant voltage acrosssaid pick-up inductance, and means for continuously varying one of saidvoltages to produce a sweep voltage in the output of said rectifyingmeans.

6. A radio frequency spectroscope as set forth in claim l, wherein saidganging means is r0- tatable. and the capacitances of both of saidcapaci-tors vary from minimum to maximum and then back to minimum aplurality of times in a single rotation of said gauging means.

7. A radio frenquency spectroscope for analyzing noise and signal energycomprising a radio frequency oscillator having a Variable capacitorl forcontinuously scanning said oscillator in frequency within a prescribedband, a converter adapted to heterodyne said energy with the output ofsaid oscillator, a Xedly-tuned network responsive to the output of saidconverter, means for rectifying the output of said network, acathode-ray oscilloscope, means responsive to the rectied output of saidnetwork to deect the beam Yof said oscilloscope along one coordinate,means to deflect the beam of said oscilloscope along a secondcoordinate, and sweep voltage means coupled to said last named means andcomprising a second radio frequency oscillator having a tank inductance,a pick-up inductance inductively coupled to said tank inductance, asecond variable capacitor capacitively coupling said tank inductance tosaid pick-up inductance to develop a voltage thereacross in phase oppo-EVERETT R. MCC`OPPIN REFERENCES CITED The following references are ofrecord inthe file of this patent:

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